This application is based on Japanese Patent Application No. 2001-212268 filed on Jul. 12, 2001, the disclosure of which is incorporated herein by reference.
The present invention relates to an evaporative emission control system in which a canister is provided in an evaporative gas purge system connecting an air intake pipe of an engine and a fuel tank.
In an evaporative emission control system disclosed in JP-A-2000-345934 and JP-A-11-343927, for example, a negative pressure generated in an air intake pipe during an engine operation is introduced into a fuel tank. An evaporative gas purge system connecting the air intake pipe and the fuel tank is then maintained under the negative pressure. Change of the pressure is sensed by a sensor, thereby determining leaks in the purge system.
In this kind of evaporative emission control system, the leak check is executed when the engine is running. While a vehicle is moving, a fuel level in the fuel tank is unstable due to vibration of the vehicle. It causes changes of the pressure during the leak check. Therefore, it is difficult to accurately determine the leak when the engine is running. Because the purge system is negatively pressurized in a short time, a large amount of fuel vapor is introduced into the engine at a time. It is likely to affect an air-fuel ratio control, thereby worsening the emission.
The present invention is made in view of the above problem, and it is an object of the present invention to provide an evaporative emission control system capable of determining leaks accurately.
It is another object of the present invention to provide an evaporative emission control system that suppresses emission from becoming worse.
It is further another object of the present invention to provide a method to accurately determine a leak in an evaporative emission control system.
In an evaporative emission control system of the present invention, a canister communicates with a fuel tank through a negative pressure intake line and an air intake pipe of an engine through a purge line. Fuel vapor generated in the fuel tank is temporarily stored in the canister, and sucked into the air intake pipe by negative pressure generated in the air intake pipe. The negative pressure in the air intake pipe is introduced into the fuel tank through the negative pressure intake line. The purge line is provided with a purge valve to open and close the purge line.
After the engine is switched off, the purge valve is closed and defines a closed space between the purge valve and the fuel tank. The negative pressure introduced in the fuel tank is introduced into the closed space. After the closed space is maintained under a uniform negative pressure, a leak check of the closed space is started.
Because the leak check is executed when the engine halts, that is, when a fuel level in the tank is stable, the leak is accurately detected. Further, because the fuel tank is maintained under the negative pressure when the engine is running, the fuel vapor is prevented from leaking outside through the fuel tank, canister and the like.